This likely works for even older versions but I have only tested on 8,9,10 (11,12). It's quite impressive at how easy it is to upgrade from a very old version to the new version. I would say that Debian version upgrades are some of the quickest and smoothest of any distro.
1.) Backup your /etc/apt/sources.list
cp /etc/apt/sources.list ~
#Get your keys first, you need the latest keys for Debian 11 or it won't work:........